1. Field of the Invention
The invention relates generally to sputtering of materials. In particular, the invention relates to the auxiliary magnets used in the sputtering of magnetic materials.
2. Background Art
Magnetic random access magnetic memory (MRAM), also called magnetoelectronic memory, is receiving increased interest and is expected to shortly enter commercial manufacture. MRAM as currently conceived involves the integration of magnetic materials into semiconductor integrated circuits to produce chips having millions of memory cells on which information can be written and read. When it is fabricated on a silicon wafer, silicon support circuitry can be included on the same wafer. Importantly, the MRAM is non-volatile memory. In a write operation, the magnetic material is poled into one of two magnetic states. In a read operation, the magnetic state of the poled material is determined. The magnetic state of the memory is maintained in the quiescent period between the write and read operations with no power being applied to the memory cell.
Many forms of MRAM have been contemplated, some of which are reviewed by Johnson in “Magnetoelectronic memories last and last . . . ,” IEEE Spectrum, vol. 37, no. 2, February 2000, pp. 33-40. One form includes a magnetic tunneling junction (MTJ), which is explained in more detail by Parkin et al. in “Exchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory,” Journal of Applied Physics, vol. 85, no. 8, 15 Apr. 1999, pp. 5828-5833. FIG. 1 is a schematic cross-sectional view of one MTJ cell 10 in a large two-dimensional array. The many cells 10 are formed by fairly standard techniques well developed for the most part in the semiconductor integrated circuit industry. Furthermore, when the magnetic cell is fabricated on a silicon wafer, silicon support circuitry can be integrated on the same wafer as the magnetic memory.
The magnetic storage cell 10 is centered on a junction structure 12 including a fixed magnetic layer 14 and a free magnetic layer 16 separated by a very thin non-magnetic tunneling barrier 18. Both magnetic layer 14, 16 are relatively thin, typically on the order of 1 to 20 nm thick. In the most prevalent MRAM design, the magnetic layers 14, 18 are electrically conductive, and the tunneling barrier 18 is a very thin electrically insulating layer, typically on the order of less than 2 mn or even 1 nm. The extreme thinness allows quantum mechanical electron tunneling through the otherwise dielectric barrier 18. An alternative structure replaces the dielectric barrier with a metal barrier through which spin can tunnel, as described by Tehrani et al. in “High density submicron magnetoresistive random access memory,” Journal of Applied Physics, vol. 85, no. 8, 15 Apr. 1999, pp. 5822-5827. Somewhat similar magnetic stacks may be used to form spin valves or spin transistors, as Zorbette describes in “The quest for the spin transistor,” IEEE Spectrum, vol. 38, no. 12, Dec. 2001, pp. 30-35.
The two magnetic layers 14, 16 of the MTJ cell 10 are distinguished in that the fixed magnetic layer 14 has a predetermined magnetization, for example, in one of the two horizontal directions of the illustrations, while the free magnetic layer 16 can be semi-permanently poled and repoled into either of the two horizontal directions. Which horizontal direction determines the state of the memory cell 10. The magnetic layers 14, 16 are typical composed of transition metals and their alloys, for example, NiFe, CoFe, Co, or Ru or a bilayer or sandwich structure of such materials. The iron alloys are typically rich in the transition metal, for example, Ni80Fe10 or Co90Fe10. The barrier 18 may be composed of oxidized aluminum. The distinction between fixed and free magnetic layers may be determined by the fixed layer being grown on an anti-ferromagnetic layer, also called the exchange-bias layer which prevents the adjacent fixed magnetic layer from changing state. The exchange-bias layer is typically a manganese alloy, for example, Pt50Mn50. Other anti-ferromagnetic compositions include MnFe, MnIr, MnRh, NiO, TbCo, and iridium alloys. The exchange-bias layer allows the two magnetic layers 14, 16 to have the same composition. Other buffer, transition, and capping layers are typically included in the stack structure.
In the illustrated MTJ cell 10, a metallic bit line 20 is electrically connected to the free magnetic layer 14 of the storage structure 12 as well as to many other cells 10 in the plane of the illustration. The fixed magnetic layer 14 is electrically connected to a conductive cross connector 22 electrically selected by an isolation transistor 24. In this embodiment, it is assumed that the isolation transistor 24 is individually selected for each memory cell 10. A digit line 26 underlies the storage structure 12 and runs orthogonally to the bit line 18 in the two-dimensional memory array.
The operation of the memory cell 10 relies upon the effect that the impedance of the storage structure 12 when the two magnetic layers 14, 16 are aligned to be parallel, as illustrated in the schematic illustration of FIG. 2, is significantly less than the impedance when the two magnetic layer 14, 16 are aligned to be anti-parallel, as illustrated in the schematic illustration of FIG. 3. The impedance depends upon quantum mechanical spin effects between the two magnetic layers 14, 16 and may be measured as either voltage or current by measuring circuitry gated by the isolation transistor 24, usually in comparison to a reference cell. The measuring electrical path proceeds from the bit line 20 through the storage structure 12, cross connect 22 and isolation transistor 24.
It is possible to initially pole the fixed layer 14 (as well as the free layer 16) with a large current pulse and thereafter in operation use lesser currents to switch only the free layer 16. However, for a large dense memory, the one-time poling of the fixed layer 14 significantly complicates the chip design. It is much preferred that the magnetization direction of the fixed magnetic layer 14 be impressed during the growth of the fixed layer 14 and that in operation the fixed layer 14 remain permanently polarized. On the other hand, during operation the magnetization direction of the free magnetic layer 16 can be written and rewritten in selected directions according to orthogonal currents passed through the bit line 20 and the digit line 26. Once the magnetic state is written into the storage cell 10, it remains until rewritten. Whatever state is currently written in the storage cell 10 is read by measuring the impedance of the storage structure 12. It is important that the magnetization direction impressed during growth of the fixed magnetic layer 14 be properly aligned with the bit and digit lines 20, 26. Any significant misalignment, for example, more than 5 or 10°, degrades the impedance differential between the two memory states.
The magnetic layers 14, 16 are conveniently formed of a magnetic metal such as NiFe or CoFe by a sputtering process using a sputtering target of approximately the same composition. Sputtering has the advantage of a relatively high sputtering rate using relatively expensive source materials in a relatively simple sputter reactor. However, sputtering of the stated magnetic layers presents some difficulties. Usually magnetron sputtering is employed in which a magnetron is positioned in back of the target to create a magnetic field in front of the target to increase the plasma density and hence the sputtering rate. However, the magnetic characteristics of the target inhibits the effect of the magnetron.
A more difficult problem arises when it is desired to deposit the fixed magnetic layer 14 with its magnetization fixed in a predetermined direction determined by the bit and digit lines 20, 26. In the past, such selective magnetization during deposition has been accomplished by placing coils or other means outside the sidewalls of the deposition chamber. The magnetic direction of the magnetic means determines the magnetization of the magnetic metal being deposited. However, these prior approaches of placing the magnetic means outside the chamber suffer from excessive size and further require a redesign of the vacuum chamber and its ports.
Sekine et al. in U.S. Pat. No. 5,660,744 have suggested the use of a Halbach magnet array, also called a magnetic dipole ring, for creating a uniform magnetic field for confining and intensifying a plasma, primarily in the context of reactive ion etching although with brief reference to sputtering. Such magnet arrays will be described in detail later. However, the Sekine implementation suffers from the disadvantage of being positioned outside the vacuum chamber and hence requiring a significant chamber redesign to incorporate a magnet into a sputtering reactor system already designed for non-magnetic sputtering. Miyata in U.S. Pat. No. 5,519,373 more explicitly refers to the use of a magnetic dipole ring in a sputter reactor as the principal magnetron for creating a high density plasma. It too suffers from being located outside the vacuum chamber.